The heart of any ultrasound (imaging) system is the transducer which converts electrical energy in acoustic energy and back. Traditionally these transducers are made from piezoelectric crystals arranged in linear (1-D) transducer arrays, and operating at frequencies up to 10 MHz. However, the trend towards matrix (2-D) transducer arrays and the drive towards miniaturization to integrate ultrasound (imaging) functionality into catheters and guide wires has resulted in the development of so called capacitive micro-machined ultrasound transducer (cMUT) cells. These cMUT cells can be placed or fabricated on top of an ASIC (Application Specific IC) containing the driver electronics and signal processing. This will result in significantly reduced assembly costs and the smallest possible form factor.
A cMUT cell comprises a cavity underneath the cell membrane. For receiving ultrasound waves, ultrasound waves cause the cell membrane to move or vibrate and the variation in the capacitance between the electrodes can be detected. Thereby the ultrasound waves are transformed into a corresponding electrical signal. Conversely, an electrical signal applied to the electrodes causes the cell membrane to move or vibrate and thereby transmitting ultrasound waves.
An important question with cMUT devices is how to reduce or suppress acoustic coupling of the ultrasound waves (or reverberation energy) to the substrate. In other words it is a question how to minimize undesired substrate interactions (such as reflections and lateral cross-talk) or coupling.
Another question is how the cMUT device is connected to the ASIC. There are multiple ways, in particular three general ways, how the connection between a cMUT device and an ASIC may be realized. FIG. 1a-c show the three different solutions of a cMUT device connected to an ASIC. The first solution shown in FIG. 1a is to place a separate cMUT device (substrate 1 and cMUT cells 3) on top of the ASIC 4 and use wire bonds 5 for the connections. This first solution is the most flexible and simplest solution. However, this solution is only attractive for linear arrays.
For 2D arrays the large number of interconnects between each cMUT device and the driving electronics makes it necessary to place each cMUT device directly on top of the driving electronics. The second solution is thus to process the cMUT cells 3 as a post processing step on top of an already processed ASIC 4, as shown in FIG. 1b. This yields a so-called “monolithic” device (one chip) where the cMUT cells are fabricated directly on top of the ASIC. Such “monolithic” devices are the smallest, thinnest devices and have the best performance in terms of added electrical parasitics. However, with this solution, in order to minimize undesired substrate interactions (such as reflections and lateral cross-talk), significant substrate modifications to the substrate underneath the cMUT cell may be required. These modifications may be at the worst case impossible on a CMOS substrate, or at the best case very difficult to implement because it may require process steps and/or materials which are incompatible with the technologies available or allowed in the foundry in which the combination of the cMUT device and the ASIC is fabricated. Compromises would have to be made that lead to suboptimal performance. Another challenge with this second solution of monolithic integration is that the ASIC process and the cMUT process are tightly linked, and that it will be difficult to change to e.g. the next CMOS process node.
A third, alternative solution is to use a suitable through-wafer via hole technology to electrically connect the cMUT cells 3 on the front side of the substrate 1 to contacts on the backside of the substrate 1, so that the substrate or device can be “flip-chipped” (e.g. by solder bumping) on the ASIC 4 (see FIG. 1c). This yields a so-called “hybrid” device (two chips) which comprises the cMUT device and the ASIC.
In one example the cMUT cells are fabricated with or in the substrate, thus with the same technology as the substrate. Such a cMUT device is for example disclosed in US 2009/0122651 A1. However, such device and/or its method of manufacturing needs to be further improved.